Worst mass shooter in U.S. history - NYC-born 'Islamic extremist' Omar Mateen, 29, who shot dead 50 and injured 53 in Orlando gay club massacre was homophobe 'who got angry when he saw two men kissing'

The suspected Islamic extremist who slaughtered at least 50 people after taking party-goers hostage inside a gay club in Orlando was 'angry after seeing two men kissing'.

Law enforcement sources have identified the shooter, who was wielding an AR-15 assault rifle and a handgun, as US citizen Omar Mateen, 29, from Port St. Lucie in Florida.  

The gunman, who was born to Afghan parents, was carrying a suspicious device, possibly a suicide vest, when he opened fire on the dance floor inside Pulse in the early hours of this morning.

At least 50 people were killed and 53 others were injured in the shooting in the deadliest mass shooting in US history. A surgeon at Orlando Regional Medical Center said the death toll was likely to climb.

An FBI spokesman said the mass shooting is being investigated as an act of terrorism, adding that they are looking into whether this was an act of domestic or international terror, and if the shooter was a lone wolf. 

The FBI believe the gunman, who was shot dead by officers, may have 'leanings to radical Islamic terrorism'.  

But his father, Mir Seddique, told NBC News his son became angry when he saw two men kissing in Miami several months ago.

'This has nothing to do with religion,' he said. 'We are saying we are apologizing for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country.'

Writing on Facebook, Pulse, which described itself as the most prominent gay club in Orlando, urged party-goers to 'get out and keep running' as bullets started flying at around 2am local time.

Orlando Police Chief John Mina said the suspect exchanged gunfire with an officer working at the club around 2am, then went back inside and took hostages. There were about 320 people inside the club at the time of the shootings and about 100 people were taken hostage. 

At around 5am authorities sent in a SWAT team to rescue the hostages. Nine hero officers used a 'controlled explosion' to distract the shooter before fatally shooting him and were able to rescue about 30 hostages who were hiding in the bathroom of the club.

During the gunfire, an officer was shot, but he was saved by his helmet. It was thought that at least one hostage had been locked in a bathroom with gunshot wounds. 

Mayor Buddy Dyer said in a press conference: 'Many were saved by the heroic efforts of the men and women of the OPD, the Orange County Sheriffs, Seminal County Sheriff's office.' 

At around 6am local time police tweeted: 'Pulse Shooting: The shooter inside the club is dead.'

Twitter accounts which claim to be affiliated to the Islamic State have hailed the shooting, although DailyMail.com cannot confirm whether the terror group was behind the attack. 

U.S. Rep. Alan Grayson identified the shooter as Mateen, citing law enforcement officials in speaking to reporters.

Mateen, a Muslim and father to a three-year-old son, was born in 1986 in New York and married Sitora Alisherzoda Yusufiy, who was born in Uzbekistan, in 2009.

His father told NBC News:  'We were in Downtown Miami, Bayside, people were playing music. And he saw two men kissing each other in front of his wife and kid and he got very angry.

'They were kissing each other and touching each other and he said, "Look at that. In front of my son they are doing that". And then we were in the men's bathroom and men were kissing each other.'

Seddique said Mateen had a job in security and attended Indian River State College, as well as having an associates degree in criminal justice.

He was 'on the radar' of U.S. officials for some time, law enforcement officials told ABC News. However he was not the target of a specific investigation. 

Manteen had a Statewide Firearms License and was trained in firearms, according to Fox News. Assistant FBI Agent in Charge Ronald Hopper said Mateen made 'threats in the past that he has ties to terrorist organizations'. 

Several ISIS-linked Twitter accounts have praised Mateen's actions, although there has been no official claim of responsibility.  

Police were at his apartment in Port St. Lucie this morning. Ken Mascara, Sheriff of Port St. Lucie County, told DailyMail.com: 'We have sealed the apartment where Mateen was living. The FBI is bringing bomb diffusing equipment.' 

Jon Alamo said he was inside the club, which was hosting a Latin-themed night, when a man holding a weapon entered the room he was in.

'I heard 20, 40, 50 shots,' Alamo said. 'The music stopped.'

Club-goer Rob Rick said it happened just before closing time. 'Everybody was drinking their last sip,' he said.

He got on the ground and crawled toward a DJ booth.

A bouncer knocked down a partition between the club area and an area in the back where only workers are allowed. People inside were able to then escape through the back of the club.

Christopher Hansen said he was in the VIP lounge when he started hearing gunshots.

'I was thinking, are you kidding me? So I just dropped down. I just said please, please, please, I want to make it out,' he said. 'And when I did, I saw people shot. I saw blood. You hope and pray you don't get shot.' 

He continued to hear shooting even after he emerged, where police were telling people to back away from the club. He saw injured people being tended to across the street.

Mina Justice was outside the club trying to contact her 30-year-old son Eddie, who texted her when the shooting happened and asked her to call police. 

He told her he ran into a bathroom with others to hide. He then texted her: 'He's coming.' 

She said: 'The next text said: "He has us, and he's in here with us". That was the last conversation.'